<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    *{
      margin: 0;
    }
    .box1{
      width: calc(100px - 50px);
      background-color: red;
    }
  </style>
</head>
<body>
<div class="box1">1</div>
</body>
</html>